1. Field of the Invention
The present invention relates to a method and an apparatus for depositing a film substantially at atmospheric pressure. The invention is, therefore, capable of offering an inexpensive film formation apparatus which does not need any evacuating apparatus such as a vacuum pump. The film can be made from hard carbon, silicon nitride, silicon oxide, or other similar material. Films of this kind can be used to harden or improve the surface of plastic, glass, or organic photosensitive body or to prevent reflection at such surface. These films can find wide application. The invention is intended to provide a method and an apparatus for mass-producing these films at low costs.
2. Prior Art
Presently, coatings made of materials having unconventional functions such as hard carbons, silicon nitride, and silicon oxide are often formed by plasma chemical vapor deposition (PCVD). Most PCVD processes utilize reduced pressure. The main advantages of the use of reduced pressure are: (1) The effects of impurities contained in the atmosphere such as oxygen can be eliminated; (2) Where a plasma is employed, a stable glow discharge is obtained over a wide region; (3) Since the mean free path is long, the film thickness uniformity and the step coverage can be easily improved. However, an expensive evacuating apparatus and a vacuum vessel that is strong enough to withstand vacuum are needed to obtain reduced pressure (vacuum).
Generally, even a trace amount of impurity contamination is not tolerated in the field of semiconductor technologies since the coatings must have high performance. High depreciation costs of equipment can be easily assigned to commercial products having high added values. For these reasons, these coatings have been fabricated by plasma chemical vapor deposition as described above. On the other hand, where coatings are formed for hardening or improvement of surface of plastic, glass, or organic photosensitive body, or for prevention of reflection at such surface, very high purities are not required. Rather, increased costs due to the usage of expensive equipment present problems. That is, the best compromise must be struck between performance and cost.
Plasma CVD processes that need no evacuating apparatus are known. A plasma CVD which is applied to etching is described in Japanese Patent Application No. 286883 filed in 1990. In particular, a space is filled with a flowing gas consisting mainly of helium at a pressure close to atmospheric pressure. An AC electric field is applied to the space to ionize the gas and a halogenated etching gas added to it. In this way, excitons are produced and used for etching. Also, a technique making use of an electric discharge of a gas consisting mainly of helium for deposition of a thin film is known (the 2nd Volume of the Manuscripts for the 37th Japanese Applied Physics-Related Combined Lecture Meeting, 28p-ZH-10). However, in this technique, the reaction space is required to be replaced with a gas consisting mainly of helium and, therefore, the space must be once evacuated to a vacuum.
As described previously, the conventional coating formation process at reduced pressure is too expensive only for the hardening of the surface of plastic, glass, or organic photosensitive body or for formation of a coating. Therefore, more economical methods are being sought. A contemplated, economical coating formation method exploits an electric discharge at atmospheric pressure. This method yields the following advantages: (1) Since vacuum evacuation is not needed, an expensive evacuating system is not necessitated; (2) The time heretofore taken to evacuate the space can be omitted and so the processing time can be shortened; and (3) Since the coating is formed at high pressure, the collision time is short and the reaction rate is high, so that the processing time can be shortened. These can make the coating formation apparatus cheaper and shorten the processing time and hence contribute greatly to reductions in the costs of coating formation.
Formation of a coating at atmospheric pressure poses three problems. The first problem is contamination with atmospheric components. Specifically, ions, radicals, and other matter are produced in the space in which an electric discharge is induced. When they are being transported to the surface of the substrate on which the coating is formed, they react with impurities in the atmosphere, especially oxygen, thus affecting the coating. Since the surface on which a coating is being formed is active, the impurities adhering to the surface such as oxygen will deteriorate the performance of the coating.
The second problem is that the electric discharge space is limited to a narrow region. Generally, where a coating is formed on a substrate, it is required that the coating be created uniformly over a large area. For this purpose, a plasma must be generated over a wide area. Where an electric discharge is produced at atmospheric pressure, the mean free path of particles is as short as less than 1 .mu.m. Therefore, recombination due to collision of electrons with ions in the space takes place frequently. Sometimes, the electric discharge region cannot be usually extended to over several millimeters.
The third problem is that the reaction rate is too high. In particular, ions and radicals collide with high probability and so reactions occur inside the space before the coating is grown on the substrate surface. As a result, they deposit as powder on the substrate.